In view of preservation of the global environment, automobile emission control has increasingly been advanced. In particular, diesel engines designed to be mounted on vehicles have been required to decrease particulate matters (PMs) and nitrogen oxides (NOx). Diesel particulate filter devices (DPF devices) have been used to decrease the PMs, and urea NOx selective reduction catalyst devices (urea SCR devices), hydrocarbon NOx selective reduction catalyst devices (HC-SCR devices), lean NOx trap catalyst device (LNT devices), and the like are used to decrease the nitrogen oxides. Removal of hazardous substances by mounting these multiple types of exhaust gas purification devices has been advanced.
As one such example, an exhaust gas purification system as described in Japanese patent application Kokai publication No. 2010-242515 has been proposed. In the system, an oxidation catalyst, a urea injection device, a diesel particulate filter device, a NOx selective reduction catalyst converter, and an oxidation catalyst are disposed in an exhaust passage in this order from an upstream side. Also, a urea decomposition catalyst is supported in the diesel particulate filter device, instead of supporting a catalyst with an oxidizing function.
Moreover, there has also been an exhaust gas purification system 1X as illustrated in FIG. 17 including an exhaust gas purification apparatus 20X in which an oxidation catalyst device 21, a diesel particulate filter device (DPF) 22, and a NOx selective reduction catalyst device (SCR) 23X are disposed in this order from an upstream side at a position downstream of a turbine 14 of a turbocharger provided in an exhaust passage 13 of an internal combustion engine 10, and a urea injection nozzle 25 is provided between the diesel particulate filter device 22 and the NOx selective reduction catalyst device 23X.
With the improvement in engine combustion, fuel consumption has been improved and the total amount of discharge of particulate matters and nitrogen oxides has been decreased as well. On the other hand, the temperature of exhaust gas that flows into the exhaust gas purification apparatus has been decreased. Specifically, as a result of the improvement in engine's combustion conditions, the temperature of exhaust gas has been decreased by 30° C. to 50° C. or greater as compared to conventional cases. In addition, exhaust gas purification apparatuses have been employing multiple devices therein and therefore increased in size, thus increasing the thermal capacity. For these reasons, it has become difficult to ensure the activation temperatures of the catalysts.
In addition, in a urea SCR system, it is difficult to shorten the distance from a urea feeder such as a urea water injection nozzle to a urea NOx selective reduction catalyst device in view of evenly dispersing urea water and accelerating decomposition of the urea to ammonia. This has also been a major cause of the increase in the size of exhaust gas purification apparatuses.
As a countermeasure to these problems, the inventor of the present application has proposed a diesel engine exhaust gas purification apparatus as described in Japanese patent application Kokai publication No. 2011-149400, for example. In the apparatus, previous-stage oxidation catalysts (DOCs), a urea injection nozzle, a turbine of a turbocharger (low-pressure stage turbine), a diesel particulate filter (DPF), a selective reduction catalyst (urea SCR), and a subsequent-stage oxidation catalyst (R-DOC) are disposed in an exhaust passage in this order from an upstream side. With this configuration, each post-treatment unit is made closer to exhaust ports, so that the heat of the exhaust gas is effectively utilized for the each post-treatment unit easily to ensure its catalyst activation temperature.
Meanwhile, the modes for measuring exhaust gas will switch from the conventionally used JE05 driving mode (Japanese driving mode simulating inner-city driving), the NEDC (European driving cycle) driving mode, and the like to a world harmonized standard, or WHDC (vehicle cycle for testing exhaust gas emission of heavy vehicles) driving mode, and the like. For this reason, decreasing exhaust gas during a cold mode and under high-temperature, high-flow-rate conditions will be necessary.
On the other hand, as for urea SCR systems, controlling adsorption of urea, its intermediate, and ammonia (NH3) has been considered for improving the nitrogen oxide (NOx) removal rate at low temperatures. However, there is a problem in that controlling adsorption of these is difficult in high-temperature, high-flow-rate ranges. Moreover, as for DPF systems, the decrease in the temperature of exhaust gas passing through a DPF device decreases the range where continuous regeneration is possible. This increases the frequency of exhaust heating control performed to forcibly combust particulate matters (PMs) captured by the DPF device. Thus, a problem arises in that the amount of carbon dioxide (CO2) discharged during forced regeneration of the DPF device increases.